Index: llvm/trunk/include/llvm/Support/GenericDomTree.h
===================================================================
--- llvm/trunk/include/llvm/Support/GenericDomTree.h
+++ llvm/trunk/include/llvm/Support/GenericDomTree.h
@@ -522,7 +522,9 @@
   ///
   /// Batch updates should be generally faster when performing longer sequences
   /// of updates than calling insertEdge/deleteEdge manually multiple times, as
-  /// they can reorder the updates and remove redundant ones internally.
+  /// it can reorder the updates and remove redundant ones internally.
+  /// The batch updater is also able to detect sequences of zero and exactly one
+  /// update -- it's optimized to do less work in these cases.
   ///
   /// Note that for postdominators it automatically takes care of applying
   /// updates on reverse edges internally (so there's no need to swap the
Index: llvm/trunk/include/llvm/Support/GenericDomTreeConstruction.h
===================================================================
--- llvm/trunk/include/llvm/Support/GenericDomTreeConstruction.h
+++ llvm/trunk/include/llvm/Support/GenericDomTreeConstruction.h
@@ -1122,6 +1122,22 @@
   //~~
 
   static void ApplyUpdates(DomTreeT &DT, ArrayRef<UpdateT> Updates) {
+    const size_t NumUpdates = Updates.size();
+    if (NumUpdates == 0)
+      return;
+
+    // Take the fast path for a single update and avoid running the batch update
+    // machinery.
+    if (NumUpdates == 1) {
+      const auto &Update = Updates.front();
+      if (Update.getKind() == UpdateKind::Insert)
+        DT.insertEdge(Update.getFrom(), Update.getTo());
+      else
+        DT.deleteEdge(Update.getFrom(), Update.getTo());
+
+      return;
+    }
+
     BatchUpdateInfo BUI;
     LegalizeUpdates(Updates, BUI.Updates);